His or her ex.

'son' - singular possessive pronoun for a male or female 'owner' (noun gender) of a masculine 'possession' (noun gender; or also for vowel or silent h commencing noun of either gender). I agree with you that it could be either his or her ex, without context. The translation would not be wrong, just one of the possibilities - in real-life the context would usually be known or clarified. In quizzes, only one of the right answers, or a multi-choice selection would be needed to determine. Common mistakes with mon/ma/mes, ton/ta/tes and son/sa/ses (French Possessive Adjectives) Let's see if others agree or not.

That doesn't answer my question - I know those rules, and it's why I'm asking!

It's «son ex» regardless of the gender of the ex or the gender of the person whose ex it is. (But even if we knew the gender of the ex, it wouldn't help.)

My question is how do we know whose ex (Il's or Maria's) is being referred to here?

Alternatively, is it impossible to know?

The use of 'her' in the translation strongly suggests the gender of the 'owner' *is* known - if the gender is unknown, then 'their' should have been used, or 'he' (if you're a traditionalist who doesn't read Shakespeare).

As I said, in answer to your question, I think it could be either when you are presented with this sentence and no context. It is not necessary to use 'their' in the translation - it tells you in this case it is Maria's ex, so the translator knew it to be 'her ex'. You asked if it could have been his ex (il) with just the sentence, and the answer is (again, I think) yes, it could have been, in which case the translation could have stated his ex, not her ex. If you don't have context, you don't know - uncommon in real life, but not uncommon on quiz questions, in which case if both are given as options, both are right, if only one, then that is right for the quiz. If your issue is beyond the translation, I am not interested in pursuing any further.

Maarten, regarding your "As I said", you seem to have added significantly to your answer since I read it. This editing is also why my response includes the bit about "regardless of...the gender of the person whose ex it is", because you originally only referred to a "female 'owner'".

I'm not sure why you're pontificating about quiz questions when I stated it was an example. It's not a quiz question, it's an example in the lesson material.

Your concept of the translator knowing the gender doesn't apply in the lesson material as later in the lesson «Tu lui rappelles son meilleur ami» is given two translations, one for his and one for her.

I think my question stands; the material seems to point to it always being the person who's reminded, as what I've seen consistently matches 'son/sa/ses' with the person  However, there's no note regarding this, I can't be sure whether it's an oversight or universally the case.

The grammar seeming to allow it doesn't clarify. For example, from the lesson you linked, «Il déteste son frère» means "He hates his brother" and would not be understood as "He hates her brother" (without further context). The lesson you linked isn't entirely explicit, so there's precedent for us having to figure this kind of thing out from the examples.

Not sure what you mean by "If your issue is beyond the translation, I am not interested in pursuing any further".